Engine exhaust purifying apparatus

ABSTRACT

The present invention has its object to judge the establishment of conditions influencing on the evaluation of performance of a NOx catalyst, whereby when being established, the evaluation of performance of the NOx catalyst is stopped to thereby prevent the performance of the NOx catalyst from being erroneously judged.  
     The aforementioned object is realized by an engine exhaust purifying apparatus comprising a NOx trap which is provided in an exhaust passage of an engine to trap NOx in exhaust gases by adsorption or absorption when the air/fuel ratio of a mixture gas is lean, and to release or reduce NOx when the air/fuel ratio is rich, and NOx trapping quantity judging means for evaluating the exhaust purifying performance including the NOx trapping quantity of the NOx trap, wherein the operating state of the NOx trap is measured directly or indirectly, and when the measured operating state is judged to be beyond a predetermined range, the purifying performance evaluation of the NOx trap is inhibited or stopped.  
     Further, the aforementioned object is realized by an engine exhaust purifying apparatus comprising a NOx trap which is provided in the engine exhaust passage to trap such as adsorption or absorption NOx of exhaust gases when the air/fuel ratio of exhaust gases is lean, and to release or reduce NOx when the air/fuel ratio is rich, and NOx trapping quantity judging means for evaluating the exhaust purifying performance including the NOx trapping quantity of the NOx trap, characterized in that the operating state of the NOx trap is measured directly or indirectly, and when the measured operating state is judged to be beyond the predetermined range, the purifying performance evaluation of the NOx trap is inhibited or stopped.  
     According to the present invention, since the release of oxygen stored and the timing of the release of NOx trapped are detected separately by the output of the air/fuel ratio sensor at the downstream of the NO trap for trapping NOx, it is possible to provide an engine exhaust purifying apparatus capable of detecting the NOx trap quantity and the oxygen storage capacity separately with good accuracy. Under the circumstances that the aforesaid detection accuracy cannot be secured, it is possible to prevent an erroneous detection by inhibiting or stopping the detecting operation.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an engine exhaust purifying apparatus.

[0002] There has been proposed technology, for improving the fuel consumption of the engine, for making air excessive (hereinafter referred to as lean) than that in a stoichiometric air/fuel ratio (hereinafter referred to as stoichiometry) to lean-burn fuel.

[0003] For example, a system for injecting fuel near an intake port of an intake pipe portion (port injection) in which the lean combustion at air/fuel ratio of about 20 to 25 is realized, and a cylinder direct fuel injection system (cylinder injection) in which a stratified mixture is formed to realize the extremely lean combustion at air/fuel ratio of 40 to 50 have been used. In these technologies, it is possible to increase the lean combustion, that is, the intake air amount to thereby reduce the pumping loss and heat loss, thus improving the fuel consumption.

[0004] However, in case of combustion at stoichiometry, HC, CO and NOx in exhaust gases can be oxidized and reduced simultaneously by a three-way catalytic converter to cleanse them, whereas in the lean combustion, the reduction of NOx is difficult because the exhaust gases are excessive in oxygen. Therefore, there has been proposed an engine exhaust purifying apparatus in which an NOx absorbent is arranged in an exhaust passage to absorb NOx when the air/fuel ratio of a mixture is lean, and to release NOx when the air/fuel ratio is rich (excessive fuel), so that the air/fuel ratio is temporarily changed from lean to the stoichiometric air/fuel ratio or rich in a predetermined period to release and reduce the NOx absorbed in the NOx absorbent.

[0005] In the exhaust purifying apparatus as described above, it is desirable, in a sense of reducing the fuel consumption and the exhaust components such as HC in the exhaust gases, to temporarily change the air/fuel ratio to the stoichiometry or rich only for a period corresponding to the quantity of NOx absorbed.

[0006] As the technology for judging the completion of release of NOx when the air/fuel ratio is temporarily changed to the stoichiometry or rich, Japanese Patent No. 2692380 (WO94/17291) has been proposed. In this patent, when the air/fuel ratio detected by an air/fuel ratio sensor mounted at the downstream of a NOx trap is changed from lean to rich after the air/fuel ratio has been switched from lean to the stoichiomtry or rich, the completion of the release of NOx is to be judged. This is based on the fact that since HC and CO in the exhaust gases flown in from the upstream are consumed for reduction of NOx until NOx absorbed in the NOx trap is released and reduced even if the air/fuel ratio at the upstream of the NOx trap becomes the stoichiometry or rich, the air/fuel ratio detected by the air/fuel ratio sensor mounted at the downstream of the NOx trap becomes somewhat lean, and the air/fuel ratio detected by the air/fuel ratio sensor becomes rich after the release and reduction of the NOx absorbed in the NOx trap have been completed.

[0007] As the similar technology, Japanese Patent Laid-Open No. 10-128058 (corresponds to U.S. Pat. No. 5,743,084) discloses the technology in which the quantity of NOx is estimated, which is trapped by a time difference until the air/fuel ratio detected by the air/fuel ratio sensor mounted at the downstream of the NOx trap is switched from lean to rich after the air/fuel ratio has been switched from lean to the stoichiometry or rich, thereby monitoring the performance of the NOx trap.

[0008] However, for example, an output waveform of the air/fuel ratio sensor mounted at the downstream of a NOx trap or a trapping device (hereinafter referred to as “NOx trap”) is affected by the oxygen storage capacity even if the quantity of NOx trapped in the NOx trap is the same. This, however, has not been taken into consideration in the aforementioned prior art.

SUMMARY OF THE INVENTION

[0009] The present invention has its object to judge the establishment of conditions influencing on the evaluation of performance of a NOx catalyst, whereby when being established, the evaluation of performance of a NOx catalyst is stopped to thereby prevent the performance of a NOx catalyst from being erroneously judged.

[0010] The aforementioned object is realized by an engine exhaust purifying apparatus comprising an exhaust component trap which is provided in an exhaust passage of an engine and having a trapping function to adsorb or absorb an exhaust component and means for evaluating the performance of said exhaust component trap, wherein when at least one of an operating state of an engine system, the operating state of said exhaust component trap, and said means for evaluating the performance of said exhaust component trap is judged to be beyond a predetermined operating range, the performance evaluation of said exhaust component trap is inhibited or stopped.

[0011] Further, the aforementioned object is realized by an engine exhaust purifying apparatus comprising a NOx trap which is provided in an exhaust passage of an engine to trap NOx in exhaust gases by adsorption or absorption when the air/fuel ratio of a mixture gas is lean, and to release or reduce NOx when the air/fuel ratio is rich, and NOx trapping quantity judging means for evaluating the exhaust purifying performance including the NOx trapping quantity of said NOx trap, wherein the operating state of said NOx trap is measured directly or indirectly, and when said measured operating state is judged to be beyond a predetermined range, the purifying performance evaluation of said NOx trap is inhibited or stopped.

[0012] According to the present invention, it is possible to provide an engine exhaust purifying apparatus in which since the release of oxygen stored and the release timing of NOx trapped are separately detected from an output of the air/fuel ratio sensor at the downstream of the NOx trap for trapping the exhaust gas component, for example, NOx, the NOx trapping quantity and the oxygen storage capacity can be separately detected with high accuracy. Under the circumstances that said detection accuracy cannot be secured, the detecting operation is inhibited or stopped to thereby enable prevention of erroneous detection.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 shows a schematic view of an engine exhaust purifying apparatus according to the present invention;

[0014]FIG. 2 is a characteristic view of an air/fuel ratio sensor;

[0015]FIG. 3 is a constitutional view of ECU;

[0016]FIG. 4 is a map of a target equivalent ratio every operating region;

[0017]FIG. 5 is a view for explaining a relationship between an output waveform of an air/fuel ratio at the downstream of a NOx trap when a NOx purge is controlled and a difference in NOx trap;

[0018]FIG. 6 is a view for explaining a judging method for an oxygen storage quantity and a NOx trapping quantity based on an output waveform of an air/fuel ratio at the downstream of a NOx trap when a NOx purge is controlled;

[0019]FIG. 7 is a view showing a relationship between T2 and the NOx trapping quantity;

[0020]FIG. 8 is a view showing a relationship between T1 and the oxygen storage quantity;

[0021]FIG. 9 is a view for explaining a judging method for an oxygen storage quantity and a NOx trapping quantity based on an output waveform of an air/fuel ratio at the downstream of a NOx trap when a NOx purge is controlled, in prior art;

[0022]FIG. 10 is a view showing a relationship between Tx and the NOx trapping quantity, in prior art;

[0023]FIG. 11 is a view for explaining the NOx purge control, the deterioration judging timing, and so on;

[0024]FIG. 12 is a flow chart for explaining a fuel control process;

[0025]FIG. 13 is a flow chart for explaining a NOx purge control process;

[0026]FIG. 14 is a flow chart for explaining a deterioration judging process;

[0027]FIG. 15 is a flow chart for inhibiting a NOx catalyst (trap) diagnosis; and

[0028]FIG. 16 is a flow chart for judging the abnormality of an engine system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] For example, when a NOx trap itself has O₂ storage capacity, or when a catalyst or the like having O₂ storage capacity is arranged at the upstream and downstream close to the NOx trap, oxygen is stored during operation at a lean operation, and when the air/fuel ratio is switched from lean to stoichiometric air/fuel ratio or rich, the oxygen is released. Accordingly, an output of an air/fuel ratio sensor mounted at the downstream of the catalyst or the like having O₂ storage capacity is affected by oxygen released therefrom.

[0030] Accordingly, where the quantity of NOx trap absorbed in the NOx trap and the trapping performance are estimated by the air/fuel ratio sensor, a great error possibly results. For example, when the oxygen storage quantity is great, when the air/fuel ratio is temporarily switched changed from lean to the stochiometric air/fuel ratio or rich, the time when the output of the air/fuel ratio sensor mounted at the downstream of the NOx trap indicates lean becomes longer. Because of this, the NOx quantity is erroneously judged to be great. Conversely, when the oxygen storage quantity is small, when the air/fuel ratio is temporarily switched changed from lean to the stochiometric air/fuel ratio or rich, the output of the air/fuel ratio sensor mounted at the downstream of the NOx trap indicates rich early. Because of this, the NOx trap quantity is erroneously judged to be small.

[0031] If the lean operation is carried out, the oxygen storage quantity reaches oxygen storage capacity in a short time, but since the oxygen storage capacity itself becomes uneven due to the deterioration or the like, the erroneous judgment as described above possibly occurs.

[0032] Further, the NOx trap having oxygen storage capacity or the catalyst and so on having oxygen storage capacity arranged at the upstream and downstream close to the NOx trap are subjected to oxidizing reaction of unburnt HC, CO based on the oxygen storage capacity. Accordingly, if the oxygen storage capacity lowers, the oxidizing and reducing reactions thereof become weaken so that Nox trap and the catalyst having the oxygen storage capacity arranged at the upstream and downstream close to the NOx trap becomes deteriorated, and it is therefore desired that the oxygen storage capacity be detected independently. Also in this case, separation from the quantity of NOx trap is necessary as described above. The performance of the NOx trap is greatly affected by the state of the NOx trap itself. For example, they are the temperature of NOx trap, the O₂ storage quantity, the performance of a three-way catalytic converter and so on. Further, the engine exhaust component (construction ratio of components), the exhaust quantity, the exhaust temperature, the air/fuel ratio and so on also exert influence on the performance of the NOx trap. Furthermore, also where the performance of sensing means for evaluating the performance of the NOx trap is deteriorated, it exerts influence on the result of evaluation for the performance of the NOx trap.

[0033] According to the present invention, judgment is made of the fact that the conditions for exerting influence on the result of evaluation for the performance of the NOx trap, and when being established, evaluation for the performance of the NOx trap is stopped to thereby prevent the performance of the NOx trap from being erroneously judged.

[0034] The embodiments of the present invention will be explained hereinafter with reference to the drawings.

[0035]FIG. 1 is a constitutional view of an air/fuel ratio control device of the engine according to one embodiment of the present invention. In the present embodiment, an illustration is given of the cylinder injection system. An intake system 23 of an engine 1 comprises an air cleaner 2, an air flow sensor 3 for detecting the quantity of intake air, a throttle valve 4 for regulating the quantity of intake air, a throttle valve driving means 5, a throttle opening-degree sensor 5 a, a swirl control valve 6, a swirl control valve driving means 7, and an intake valve 8. The swirl control valves 6 are provided directly before the intake valve 8 with respect to each cylinder and constituted for integral operation. A combustion chamber 9 of the engine 1 comprises a fuel injection valve 10 for injecting fuel into the combustion chamber 9 directly, an ignition plug 11, and a cylinder pressure sensor 12. An exhaust system 24 of the engine 1 comprises an exhaust valve 13, a first air/fuel ratio sensor 14, a NOx trap 15, and a second air/fuel ratio sensor 25. The air/fuel ratio control device further comprises a sensing plate 16 mounted on a crank shaft of the engine 1, a crank angle sensor 17 for detecting a projecting portion thereof to thereby detect a rotational speed and a crank angle, and an accelerator sensor 19 for detecting an angle of an accelerator pedal.

[0036] The detected values of the respective sensors are input into an electronic control unit (hereinafter referred to as ECU) 20, which ECU 20 detects or computes an angle of an accelerator, an intake air quantity, an engine speed (rotational speed), a crank angle, a cylinder pressure, a throttle opening-degree, etc. On the basis of results therefrom, the quantity of fuel supplied to the engine 1 and the timing are computed to output a driving pulse to the fuel injection valve 10, the opening degree of the throttle valve 4 is computed to output a control signal to the throttle valve driving means 5, and the ignition timing or the like is computed to output an ignition signal to the ignition plug 11. Further, for example, where judgment is made that the NOx trap 15 is deteriorated, a signal is output to an alarm lamp 26 for giving warning to an operator.

[0037] Fuel is fed under pressure by a fuel pump from a fuel tank not shown, and is maintained at predetermined pressure (approximately 5 to 15 MPa) by means of a fuel pressure regulator and supplied to the fuel injection valve 10. A predetermined quantity of fuel is directly injected to the combustion chamber 9 in response to a driving pulse output by the ECU 20. Operating modes of the engine 1 include a stoichiometric operation, a homogenous lean operation, a stratified operation and so on. In the homogenous lean operation, fuel is injected in the intake stroke to mix with air and burn a homogenous mixture. In the stratified operation, fuel is injected in the compression stroke to distribute fuel in stratified form into the mixture, and fuel is gathered near the ignition plug 11 (to provide a rich mixture).

[0038] Intake air regulated by the throttle valve 4 flows into the combustion chamber passing through the intake valve 8. At that time, the swirl strength is controlled by the swirl control valve 6. Normally, it is set so that in the stratified operation and in the homogenous lean operation, the swirl strength is high, and in other operations, the swirl strength is low. Particularly, in the stratified operation, fuel is not spread over the whole combustion chamber 9 but gathered near the ignition plug 11 due to the aforementioned fuel injection timing, the air flow caused by the swirl, and the shape of a cavity 22 provided in the upper surface of the piston 21.

[0039] A mixture of fuel and intake air is ignited by the ignition plug 9 and burns. Exhaust gases after combustion are discharged to the exhaust system 24 through the exhaust valve 13. The exhaust gases flow into the NOx trap 15 arranged in the exhaust system 24.

[0040] The first air/fuel ratio sensor 14 outputs a signal corresponding to concentration of oxygen in the exhaust gases at the upstream of the NOx trap 15 to enable detection of the actual air/fuel ratio from the output. On the basis of the actual air/fuel ratio detected by the first air/fuel ratio sensor 14, the air/fuel ratio of the mixture supplied so as to have a target air/fuel ratio is feedback-controlled.

[0041] The second air/fuel ratio sensor 25 outputs a signal corresponding to concentration of oxygen in the exhaust gases at the downstream of the NOx trap 15 to enable detection of the actual air/fuel ratio from the output. The quantity of NOx trapped by the NOx trap 15 is judged on The basis of the actual air/fuel ratio detected by the second air/fuel ratio sensor 25.

[0042] While in the present embodiment, as the second air/fuel ratio sensor 25, a so-called O₂ sensor is used, in which as shown in FIG. 2, the air/fuel ratio is changed suddenly in the vicinity of stoichiometry to output a binary value, the sensor is not limited thereto. For example, a so-called wide air/fuel ratio sensor may be employed in which a substantially linear output is generated according to the air/fuel ratio on the basis of concentration of oxygen in the exhaust gases.

[0043] A passage and an EGR valve not shown are provided from the exhaust system 24 to the intake system 23. Particularly, in stratified operation, a large quantity of EGR is introduced in order to suppress the generation of NOx and in order to suppress the combustion speed.

[0044]FIG. 3 shows the construction of an ECU 20. Signals 3 s, 5 s, 12 s, 14 s, 25 s, 17 s, and 19 s of the aforementioned air flow sensor 3, the throttle valve opening-degree sensor 5 a, the cylinder pressure sensor 12, the first air/fuel ratio sensor 14, the second air/fuel ratio sensor 25, the crank angle sensor 17 and the accelerator sensor 19, and a signal of a cylinder discrimination sensor 27 not shown are input into an input circuit 31. A CPU 30 reads, on the basis of a program and a constant stored in a ROM 37, the input signals through an input/output port 32 to carry out arithmetic processing.

[0045] As the results of the arithmetic processing, the ignition timing, the injector driving pulse width and timing, the throttle valve opening-degree instructions, and the swirl control valve opening-degree instructions are output from the CPU 30 to an ignition output circuit 33, a fuel injection valve driving circuit 34, a throttle valve driving circuit 35, and a swirl control valve driving circuit 36 through the input/output port 32 to execute the ignition, the fuel injection, the throttle valve opening-degree control, and the swirl control valve opening-degree control. Further, for example, where judgment is made that the NOx trap be deteriorated, an alarm lamp 26 is turned on by an alarm lamp driving circuit 37. A RAM 38 is used to store values of input signal, results of operation and so on.

[0046] On the basis of a program and a constant stored in the ROM 37, the fuel injection time Ti is calculated, for example, by the following equation, and fuel is injected from the fuel injection valve 10 and supplied to the engine 1.

Ti=K·(Qa/Ne)·TGFBA·ALPHA·Kr

[0047] wherein K is the coefficient based on the characteristic of the fuel injection valve 10 or the like, Qa is the quantity of intake air, Ne is the engine speed, TGFBA is the target equivalent ratio of a mixture to be supplied to the engine 1, and ALPHA is the feedback correcting coefficient. Kr is the air/fuel ratio correcting coefficient in the air/fuel ratio changing control (hereinafter referred to as NOx purge control) for temporarily changing the air/fuel ratio of exhaust gas from lean to the stoichiometric air/fuel ratio or rich at a predetermined period.

[0048] If the target equivalent ratio TGFBA equals 1, a mixture supplied to the engine 1 is stoichiometric. On the other hand, if TGFBA is smaller than 1, a mixture supplied to the engine 1 is lean, and if TGFBA is larger than 1, a mixture supplied to the engine 1 is rich. The target equivalent ratio TGFBA is stored in the ROM 37 in advance, for example, as shown in FIG. 4, as a map of the engine speed Ne and a load (for example, a target torque calculated on the basis of a signal of the accelerator sensor 19 for detecting an angle of the accelerator pedal 18). That is, in the operating area of a load lower than the solid line L, TGAF is lean; in the operating area between the solid line L and the solid line R, TGFBA equals 1, that is, stoichiometric; and in the operating area of a load higher than the solid line R, TGFBA is larger than 1, that is, rich. Further, in the operating area of a load lower than the solid line L, and in the operating area of a load lower than the dotted line S, a stratified mixture is formed to realize the combustion by a mixture which is extremely lean, at the air/fuel ratio of 40 to 50 (stratified lean operation). In the operating area between the solid line R and the dotted line S, combustion of a mixture which is homogeneous and lean, at the air/fuel ratio of 20 to 25, is realized (homogenous lean operation).

[0049] In the stoichiometric operation (TGFBA=1, Kr=1), the feedback control is made so that the air/fuel ratio is correctly stoichiometric on the basis of the actual air/fuel ratio detected by the first air/fuel ratio sensor 14, and the feedback correcting coefficient ALPHA is calculated to be reflected on the fuel injection time Ti. ALPHA reduces when the actual air/fuel ratio is rich and increases when the actual air/fuel ratio is lean, and normally moves up and down about 1.0. ALPHA is fixed to a predetermined value or a learning value in the operations other than the stoichiometric operation.

[0050] In the lean operation (TGFBA<1, Kr=1), NOx in exhaust gases is trapped in the NOx trap 15. When the quantity of NOx trap assumes a predetermined quantity (in a predetermined period), TGFBA=1, Kr≧1, that is, the air/fuel ratio is switched to a state that concentration of oxygen in stoichiometric or rich is low (NOx purge control), so that NOx trapped by adsorption or absorption in the NOx trap 15 by HC or CO in the exhaust gases is reduced or reduced after release. While in the case of the cylinder injection type engine in the present embodiment, when the air/fuel ratio is switched to stoichiomeric or rich, the throttle valve 6 is mainly operated in the closing direction by the throttle valve driving means 5 to reduce the quantity of intake air and to control the quantity of fuel supplied, thereby changing the air/fuel ratio, it is to be noted that it is not limited to such a method as described above.

[0051] The NOx trap 15 is constituted so as to have both the so-called three-way catalyst performance in order to secure the NOx trapping at the time of lean and the exhaust purifying performance at the time of stoichiometry. For example, alumina is made to serve as a carrier, and alkaline metal and alkaline earth such as sodium Na, barium Ba or the like, and noble metal such as platinum Pt and rhodium Rh are carried. Further, cerium Ce having the oxygen storage capacity in order to enhance the so-called three-way performance at stoichiometry is sometimes carried. The NOx trap 15 adsorbs or absorbs the NOx for trapping when the air/fuel ratio of exhaust gases flown in is lean, and the NOx having been adsorbed reacts with HC or NOx in the exhaust gases and is reduced when the concentration of oxygen in the exhaust gases lowers (for example, at stoichiometric or rich). The NOx having been absorbed reacts with HC or CO in the exhaust gases, for example, by the catalyst action of platinum Pt after the NOx has been released and is reduced. In this manner, the quantity of NOx released into the atmosphere can be reduced. Further, in the stoichiometric operation, HC and Co in the exhaust gases are oxidized, for example, by the catalyst action of platinum Pt, and NOx is reduced, thus enabling the reduction of the exhaust gas components. Some Nox traps have the effect of reducing a part of NOx by HC or CO in the exhaust gases even if the air/fuel ratio of the mixture flown in is lean, depending on the kind of the NOx trap.

[0052] As described above, when the air/fuel ratio of the mixture is lean, NOx is trapped in the NOx trap. However, there is a limit in the NOx trapping capacity of the NOx trap. If the NOx trap traps Nox until the trapping capacity becomes saturated, NOx can not be absorbed any longer, and NOx passes through the NOx trap and is released to the atmosphere. Therefore, it is necessary to release NOx from the NOx trap 15 before the NOx trapping capacity of the NOx trap 15 becomes saturated. So, it is necessary to estimate how much NOx is trapped in the NOx trap 15. Next, a method for estimating the quantity of trapping NOx of the NOx trap 15 will be explained below.

[0053] If the quantity of NOx (per unit time) in the exhaust gases discharged from the engine 1 increases, the quantity of NOx (per unit time) trapped by adsorption or absorption in the NOx trap 15 also increases. Since the quantity of NOx (per unit time) in the exhaust gases discharged from the engine 1 is substantially determined based on the engine speed of the engine 1 and the load, the quantity of NOx (per unit time) trapped in the NOx trap 15 is a function of the engine speed of the engine 1 and the load. Accordingly, the quantity of NOx (per unit time) NOAS trapped in the NOx trap 15 is measured in advance as a function of the engine speed of the engine 1 and the load and stored in advance in the ROM 37 in the form of a map.

[0054] The quantity TNOA of NOx estimated to be trapped in the NOx tap 15 can be obtained by accumulating NOAS every predetermined time as in the following equation during continuation of lean operation.

TNOA (new)=TNOA (old)+NOAS

[0055] In the present embodiment, before the quantity TNOA of NOx estimated to be trapped in the NOx trap 15 reaches the saturated trapping quantity TNOAMX, the air/fuel ratio of the mixture is temporarily made stoichiometric or rich so that NOx is released or reduced from the NOx trap 15.

[0056] Since the quantity of NOx (per unit time) NOAS trapped in the NOx trap 15 is affected where the ignition timing and the fuel injection time are changed, it is therefore preferable that the quantity thereof be corrected by these parameters. Further, the quantity of NOx (per unit time) trapped in the NOx trap 15 is affected also by the quantity of NOx already trapped in the NOx trap 15. Accordingly, with the quantity of NOx (per unit time) trapped in the NOx trap 15 in the state that the NOx trapping quantity of the NOx trap 15 is rarely present being NOAS, the quantity TNOA of NOx estimated to be trapped in the NOx trap 15 may be obtained, for example, by the following equation.

TNOA (new)=TNOA (old)+(1−TNOA (old)/TNOAMX)×NOAS

[0057] That is, the quantity of NOx (per unit time) trapped in the NOx trap 15 is substantially proportional to the value obtained by subtracting the quantity already trapped from the saturated trapping quantity.

[0058] Incidentally, since sulfur is contained in fuel and lubricating oil of the engine 1, a small quantity of SOx is contained in the exhaust gases of the engine 1. The SOx is also trapped in the NOx trap 15 together with NOx. Once SOx is trapped, it is hard to be released, and as the trapping quantity of SOx increases, the quantity of NOx capable of being trapped in the NOx trap 15 gradually reduces. This means that the NOx trapping capacity of the NOx trap 15 is deteriorated. The NOx trapping capacity of the NOx trap 15 may be deteriorated even by heat during used and various materials (zinc Pd, silicon Si, etc.) other than the reasons mentioned above. Accordingly, it is necessary to see how much NOx may be trapped in the NOx trap 15, that is, to detect the NOx saturated trapping quantity TNOAMX of the NOx trap 15. This matter will be explained below.

[0059] First, the method for detecting the NOx trapping quantity actually trapped in the NOx trap 15 will be explained. When the air/fuel ratio of the mixture is temporarily made stoichiometric or rich (NOx purge control) in order to release NOx from the NOx trap 15, exhaust gases which contain much unburnt HC and CO and are low in oxygen concentration are discharged from the engine 1.

[0060] When the NOx trap 15 or a catalyst or the like having oxygen storage capacity mounted at the upstream of the NOx trap 15 is arranged, oxygen stored therein is first released or reduced. When the release or reduction proceeds and oxygen concentration of the NOx trap 15 lowers, the NOx trapped is released or reduced, and at the same time, is reduced by unburnt HC, CO, etc. An example of output waveforms of the second air/fuel ratio sensor 25 at the NOx purge control is shown in FIG. 5. Curves a and b show output waveforms of the second air/fuel ratio sensor 25 when the NOx traps 15 different in oxygen storage quantity (oxygen storage capacity) are used to make the NOx trapping quantity same, the curve a and the curve b showing that the oxygen storage capacity is small and large, respectively. If the lean operation is carried out, oxygen is stored fully to the oxygen storage capacity in a short period of time, in which case, therefore, it may be considered that the oxygen storage quantity and the oxygen storage capacity are the same. Curves b and c show output waveforms of the second air/fuel ratio sensor 25 when one NOx trap 15 is used to change the NOx trapping quantity, the curves b and c showing that the NOx trapping quantity is small and large, respectively. In this case the oxygen storage quantity (oxygen storage capacity) is the same.

[0061] As shown in FIG. 6, threshold VS1 showing lean and threshold VS2 showing rich are set, T1 indicating the time between the time when an output of the second air/fuel ratio sensor 25 crosses VS1 and the start of the NOx purge control, T2 indicating that the time between the time when an output of the second air/fuel ratio sensor 25 crosses VS2 and the start of the NOx purge control. FIGS. 7 and 8 show relationships between the NOx trapping quantity and T2 and between the oxygen storage quantity and T1, respectively, at the time of the same operating condition. As will be apparent from the drawings, a substantially linear relationship is considered between T2 and the NOx trapping quantity and between T1 and the oxygen storage quantity.

[0062] In case of the NOx trap used in the experiment, it has been confirmed by the experiment that VS1 and VS2 are set to approximately 0.2 V and approximately 0.8, respectively, whereby the oxygen storage quantity and the NOx trapping quantity are detected separately. It has been also confirmed by the experiment that the timing when the output of the second air/fuel ratio sensor 25 crosses VS2 is the timing that terminates the release of NOx trapped in the NOx trap. Accordingly, the purge control is to terminate after the output of the second air/fuel ratio sensor 25 crosses VS2.

[0063] If the second air/fuel ratio sensor 25 is deteriorated, the voltage values of VS1 and VS2 become changed. Therefore, it is preferable, for example, that the voltage values of VS1 and VS2 be corrected according to the output at the lean operation and the output at the rich operation.

[0064] It is apparent from the foregoing that even only the VS1 is set, the oxygen storage quantity can be detected from T1.

[0065]FIG. 9 shows a method for detecting the NOx trapping quantity according to prior art. Threshold VSx (approximately 0.5V) showing the vicinity of stoichiometry is set to measure the time Tx between the time when the output crosses VSx and the start of the NOx purge control. In this case, a relationship between the NOx trapping quantity and Tx is as shown in FIG. 10. If the oxygen storage quantity is constant, the NOx trapping quantity can be detected from Tx, but if the oxygen storage quantity is different, the NOx trapping quantity can not be detected accurately from Tx.

[0066] Since NOx trapped by adsorption or absorption in the NOx trap 15 is released or reduced substantially during the T2, if the NOx quantity released or reduced during that period of time is obtained, the NOx quantity being trapped in the NOx trap 15 is known.

[0067] Incidentally, when NOx is being released or reduced from the NOx trap 15, unburnt HC and CO contained in the exhaust gases are used to reduce NOx. Accordingly, the quantity NODS of NOx released or reduced from the NOx trap 15 per unit time is proportional to the quantity of unburnt HC and CO supplied per unit time, that is, the surplus fuel quantity. The surplus fuel quantity Qfex supplied per unit time is expressed by the following equation.

Qfex=k1·Ti·(Kr−1)/Kr·Ne=k1·K·Qa·(Kr−1)

[0068] wherein k1 designates proportional constant, and others designates those explained in the equation of Ti. Since the quantity NODS of NOx released or reduced from the NOx trap 15 per unit time is proportional to Qfex, if the proportional constant is k2, NODS is expressed by the following equation.

NODS=k2·Qfex=k·Qa·(Kr−1)

[0069] wherein k=k1·k2

[0070] When Kr is excessively large (air/fuel ratio is too rich) at the purge control, there is the possibility of a supply in excess of reduction reaction speed of NOx trapped in the NOx trap 15, depending on the kind of NOx trap 15. In this case, a part of unburnt HC and CO passes through the NOx trap 15, resulting in an error in calculation of the NOx trapping quantity. On the other hand, Kr at the normal NOx purge control is sometimes set to a somewhat large value (for example, Kr>1.1) in order to quicken the release or reduction of NOx. Because of this, with respect to Kr at the NOx purge control when the NOx trapping quantity is obtained, a value (for example, 1<Kr<1.1) different from that of the normal NOx purge control is preferable.

[0071] As described above, if the total sum TNOD of NODS during the period of T2 is obtained at the NOx purge control, the quantity of NOx trapped in the NOx trap 15 can be obtained. That is, the equation is as follows:

TNOD=ΣNODS (the total sum during the period of T2)=k·Σ(Qa·(Kr−1)) (the total sum during the period of T2)

[0072] In the following computing equation of the quantity NODS of NOx released from the NOx trap 15,

NODS=k·Qa·(Kr−1)

[0073] Actually, Kr is often a fixed value (for example, a plurality of fixed values are set in advance every operating mode). Accordingly, the total sum TNOD of NODS during the period of T2 is proportional to the total sum of Qa during the period of T2. From this, TNOD may be obtained by the following equation.

TNOD=k′·Qave·Kr·T2

[0074] wherein k′ designates a proportional constant, and Qave designates an average value of Qa during T2.

[0075] For detecting the NOx saturated trapping quantity TNOAMX of the NOx trap 15, the quantity TNOA trapped in the NOx trap 15 at the NOx purge control should be the NOx saturated trapping quantity. On the other hand, the normal NOx purge control is executed when the NOx quantity TNOA estimated to be absorbed in the NOx trap 15 is the value TNOAP smaller than the NOx saturated trapping quantity TNOAMX. For this reason, as shown in FIG. 11, normally, when the estimated NOx trapping quantity TNOA is TNOAP, the NOx purge control is executed, and only when the NOx saturated trapping quantity TNOAMX is detected and when TNOA is somewhat larger in value than the NOx saturated trapping quantity TNOAMX, the NOx purge control is executed. Then, the NOx trapping quantity detected value TNOD is obtained by the aforementioned method, the NOx saturated trapping quantity TNOAMX is updated according to TNOD, and further, the threshold TNOAP for starting the normal NOx purge control is also updated.

[0076] The NOx saturated trapping quantity TNOAMX of the NOx trap 15 is detected by the method as described above. Where the detected NOx saturated trapping quantity TNOAMX is smaller than a predetermined value, for example, SOx poisoning regeneration control for recovering damages caused by SOx is carried out, and where thereafter, also, the detected NOx saturated trapping quantity TNOAMX is smaller than a predetermined value, judgment is made that the NOx trap 15 is deteriorated, and the storage of a code representative of deterioration of the NOx catalyst, and/or the warning by way of lighting of an alarm lamp to an operator is executed.

[0077] The SOx poisoning regeneration control is achieved by elevating the temperature of the NOx trap 15 up to a preset temperature, for example, not less than 600° C., and making the air/fuel ratio rich to continue the operation for a predetermined period of time.

[0078] On the other hand, the estimated NOx trapping quantity TNOA includes an error since it is a estimated value to the end. The error results, for example, from a deviation between a map value in which the NOx quantity (released or reduced from the engine) trapped in the aforementioned NOx trap is preset and the actual value, or a deterioration in the NOx absorbing performance of the NOx trap 15. Accordingly, for example, it is preferable to use the estimated NOx trapping quantity TNOA by amending (correcting) it as follows. That is, the NOx trapping quantity detected value TNOD detected for the normal NOx purge control is compared with the threshold TNOAP for the estimated NOx trapping quantity TNOA for staring the NOx purge control so that the estimated NOx trapping quantity is corrected so as to be the NOx trapping quantity detected value TNOD.

[0079] More specifically, for example, the coefficient kc in the following equation is obtained, and kc is newly used for the estimated NOx trapping quantity TNOA.

kc(new)=kc(old)·TNOAP/TNOA

[0080] Alternatively, where the above-described corrected coefficient kc is greatly deviated from 1, judgment may be made that the engine 1 or the NOx trap 15 is abnormal. Specifically, where kc<1 and the deviation is great, judgment can be made that the NOx trap 15 is deteriorated. Also, preferably, for the purpose of improving the accuracy of judgment of the deterioration, when the deterioration of the NOx trap 15 is judged by kc, the aforementioned judgment of deterioration of the Nox trap is executed. Conversely, where kc>1 and the deviation is great, the quantity of NOx discharged from the engine 1 is greater than the map value preset, that is, judgment can be made that the engine is abnormal.

[0081] Preferably, the aforementioned detection of the NOx saturated trapping quantity TNOAMX and the judgment of deterioration of the NOx trap 15 are executed only when the fixed conditions are established, for example, when the temperature of the NOx trap 15 and the operating conditions are in the fixed range, or after passage of the fixed time, or when the deterioration is judged by kc as described above. The reasons will be explained below.

[0082] Since the NOx trapping quantity of the NOx trap 15 is intensely affected by the temperature of the NOx trap 15, the condition relating to the temperature of the NOx trap 15 is set. The NOx trap 15 lowers in NOx trapping quantity even if the temperature is too low or too high. The temperature may be measured directly or estimated from the operating condition.

[0083] The operating conditions are set, for example, for improving the estimated accuracy of the estimated NOx trapping quantity TNOA. Since the lean operation is continued until the estimated NOx trapping quantity TNOA assumes the NOx saturated trapping quantity TNOAMX or more, when the estimated NOx trapping quantity is estimated to be smaller than the actual value, the quantity of NOx passing through the NOx trap 15 increased as a consequence. Further, when the estimated NOx trapping quantity TNOA is estimated to be larger than the actual value, the NOx purge control starts before the NOx trapping quantity assumes the NOx saturated trapping quantity TNOAMX, possibly resulting in judgment that the NOx saturated trapping quantity TNOAMX is smaller than the actual value. For this reason, the operating area in which combustion is stable is set as the condition.

[0084] It is necessary for detecting the NOx saturated trapping quantity TNOAMX to carry out the NOx purge control after NOx has been tapped to the NOx saturated trapping quantity or more, and as a result, the quantity of NOx passing through the NOx tap 15 somewhat increases. For this reason, it is necessary to limit the frequency of detection of the NOx saturated trapping quantity TNOAMX, Specifically, execution is made after the passage of predetermined time from the previous detection of the NOx saturated trapping quantity TNOAMX, or the,frequency for execution from the start to the stop of the engine is limited.

[0085] In the above-described explanation, the NOx saturated trapping quantity TNOAMX is compared with the predetermined value for judgment of execution of the SOx poisoning regeneration control or judgment of deterioration of the NOx trap 15. On the other hand, alternatively, the following may be employed from the following equation of the NOx trapping quantity detected value TNOD used to obtain the NOx saturated trapping quantity TNOAMX, TNOD=k·Σ(Qa·(Kr−1)) (the sum total during the period of T2), or, the equation where Kr is a fixed value, TNOD=k′·Qave·T2. That is, a threshold is stored in advance in a map of Qa or Kr, and the threshold and T2 may be compared for judgment.

[0086] A further embodiment of the judgment of deterioration of the NOx trap 15 will be explained. In the normal NOx purge control, the threshold TNOAP for starting the NOx purge control is increased at a predetermined timing, for example by a predetermined value to provide TNOAPC. The respective NOx trapping quantity detected values TNOD when the threshold is TNOAP and TNOAPC, respectively, are obtained, a difference between which is calculated. When the difference is not larger than a predetermined value, TNOAP is reduced by the predetermined value. When the updated TNOAP is not larger than a predetermined value, judgment is made that the NOx trap 15 is deteriorated. The thereafter processes are similar to those in the aforementioned embodiment. In this embodiment, utilization is made of the fact that if the NOx trapping quantity is within the NOx saturated trapping quantity TNOAMX, the NOx trapping quantity also changes in reponse to the Nox quantity flowing in the NOx trap 15. Reversely speaking, if the NOx trapping quantity reaches the NOx saturated trapping quantity TNOAMX, the NOx trapping quantity in the NOx trap 15 will not increase even if any quantity of NOx flows in later. The essence of this invention lies in that a change in the NOx trapping quantity detected value TNOD when the estimated NOx trapping quantity TNOA is changed is examined to judge if it reaches the NOx saturated trapping quantity TNOAMX, thus not limiting other processes.

[0087]FIG. 12 is a flow chart showing the air/fuel ratio control process according to the embodiment. This control is started every predetermined time (for example 20 ms) from a main routine not shown.

[0088] First, in Step 100, if an area is a lean operating area is examined. Here, if the load and speed of the engine 1, the cooling water temperature, and the vehicle speed are within a predetermined range is examined. Where judgment is made that the area is not the lean operating area, the procedure proceeds to Step 113, where 1 is set to TGFBA, and 1 is set to Kr also. That is, the stoichiometric operation is carried out. Next, the procedure proceeds to Step 114, where the feedback control of the air/fuel ratio is executed on the basis of an output of the first air/fuel ratio sensor 14.

[0089] Where in Step 100, judgment is made that the area is a lean operating area, the procedure proceeds to Step 101, where the subject value (<1) is retrieved from the map of the speed and load of the engine 1 shown in FIG. 4 and set to the target equivalent ratio TGFBA. Then, the procedure proceeds to Step 102, where if a deterioration judgment request flag described later is set (=1), a deterioration judgment sub-routine (described later) in Step 115 is executed, and the control flow is terminated. If the deterioration judgment flat is not set, the procedure proceeds to Step 103. If a NOx purge request judgment flag described later is set (=1), a NOx purge control sub-routine (described later) in Step 116 is executed. Then, in Step 117, a counter CNOP for the number of times of normal NOx purge control is counted up by 1, and this control flow is terminated. If the NOx purge request judgment flag is not set, the procedure proceeds to Step 104, where feedback coefficient ALPHA=1 and air/fuel ratio coefficient Kr=1 at the NOx purge control are set. Then, the procedure proceeds to Step 105, where the fuel injection time Ti is calculated by the following equation: $\begin{matrix} {{Ti} = {K \cdot \left( {{Qa}/{Ne}} \right) \cdot {TGFBA} \cdot {ALPHA} \cdot {Kr}}} \\ {= {K \cdot \left( {{Qa}/{Ne}} \right) \cdot {TGFBA}}} \end{matrix}$

[0090] That is, the lean operation according to the target equivalent ratio TGFBA is to be executed.

[0091] In next Step 106, the estimated NOx trapping quantity TNOA is accumulated, during continuation of the lean operation, and obtained by the following equation.

TNOA (new)=TNOA (old)+kc·NOAS

[0092] Here, NOAS is calculated from the map or the like preset according to the operating condition of the engine 1. kc designates the estimated error correcting coefficient.

[0093] In next Step 107, if the counter CNOP for the number of times of the normal NOx purge control is not less than the judgment value KNOP is examined. In case of not less than KNOP, judgment is made that judgment of deterioration of the NOx trap 15 is necessary, and the procedure proceeds to Step 110. Here, if the estimated NOx trapping quantity TNOA exceeds (saturated NOx trapping quantity TNOAMX+α) is examined. In case of exceeding, in Step 111, a deterioration judgment request flag is set (=1), and the counter CNOP for the number of times of the normal NOx purge control times is cleared. In case of not exceeding, this control flow is terminated.

[0094] Where in Step 107, CNOP is less than judgment value KNOP, the procedure proceeds to Step 108 to examine the starting condition of the normal NOx purge control. Here, if the estimated NOx trapping quantity TNOA exceeds the NOx purge threshold TNOAP is examined. In case of exceeding, the NOx purge request flag is set (=1) in Step 109. In case of not exceeding, this control flow is terminated.

[0095] Judgment of deterioration is to be carried out, by the aforementioned processes, every time the normal NOx purge control is carried out KNOP times.

[0096]FIG. 13 is a flow chart showing the normal NOx purge control process according to the embodiment. When the NOx purge control request flag is set from the control flow shown in FIG. 12, it is started as a sub-routine.

[0097] First, in Step 200, the feedback coefficient ALPHA=1, and the target equivalent ratio TGFBA=1 are presented, and the air/fuel ratio correcting coefficient Kr at the NOx purge control is set. Further, control of correction of the ignition timing or the like is executed in order to reduce the shock resulting from the change in generated torque of the engine 1 caused by the change of the air/fuel ratio. Where the operating mode before the NOx purge control is started is the stratified operating mode (the extremely lean combustion operating mode whose air/fuel ratio is approximately 40 to 50 with a stratified mixture formed), there is executed further control for switching the operating mode to the homogenous operating mode (the operating mode for homogenously supplying fuel). To this end, the control of an opening degree of the swirl control valve 6, the control of the EGR quantity, and the control of changing the fuel injection timing and reducing the intake air quantity are executed.

[0098] Next, in Step 201, the fuel injection time Ti is calculated by the following equation. $\begin{matrix} {{Ti} = {K \cdot \left( {{Qa}/{Ne}} \right) \cdot {TGFBA} \cdot {ALPHA} \cdot {Kr}}} \\ {= {K \cdot \left( {{Qa}/{Ne}} \right) \cdot {Kr}}} \end{matrix}$

[0099] In next Step 202, if output Vo of the second air/fuel ratio sensor 25 exceeds VS2 is examined. If not exceeding, if Vo exceeds VS1 is examined in next Step 202. If not exceeding VS1, release or reduction of NOx is not started (stored oxygen is released or reduced), and therefore, this control flow is terminated. In case of exceeding VS1, NOx is being released or reduced, and therefore, in next Step 204, T2 is added AT (control start period) (may be added 1 by 1). Next, in Step 205, an accumulated value SQa of an air flow rate Qa and an accumulated times counter CQa are respectively updated.

[0100] Where in Step 202, Vo exceeds VS2, the release or reduction of NOx is terminated, and therefore, the procedure proceeds to Step 206 for termination process. At that time, T2 is the value obtained by measuring the time from VS1 to VS2. In Step 206, the NOx purge request flag is cleared (=0), and in next Step 207, an average air flow rate Qave during the release of NOx is calculated by the following equation.

Qave=SQa/CQa

[0101] In next Step 208, the NOx trapping quantity detected value TNOD is calculated by the following equation.

TNOD=k′·Qave·Kr·T2

[0102] In next Step 209, the estimated error correcting coefficient kc is calculated by the following equation.

Kc(new)=kc(old)·TNOAP/TNOA

[0103] In next Step 210, TNOD, TNOA, T2, SQa and CQa are initialized, and this control flow is terminated. Where the operating mode before starting the NOx purge control is the stratified operating mode, the control for switching the operating mode from the homogenous operating mode to the stratified operation is also executed, after which this control flow is terminated.

[0104]FIG. 14 is a flow chart showing the deterioration judging process according to the embodiment. When a deterioration judgment request flag is set from the control flow shown in FIG. 12, there is started as a sub-routine.

[0105] First, in Step 300, the feedback coefficient ALPHA=1 and the target equivalent weight ratio TGFBA=1 are set, and the air/fuel ratio correcting coefficient Kr at the NOx purge control is set. Further, control of correction of the ignition timing is also executed in order to reduce the shock resulting from change in the generated torque of the engine 1 caused by changing the air/fuel ratio. Note that where the operating mode prior to starting of the NOx purge control is a stratified operating mode (the extremely lean combustion operating mode at about air/fuel ratio of 40 to 50 with a stratified miture formed), there is also executed controlling of switching the operating mode to a homogenous operating mode (the operating mode for homogenously supplying fuel). For this reason, controlling such as the control of an opening degree of the swirl control valve 6, the control of EGR quantity, the change in fuel injection timing, the reduction in intake air quantity and so on is also executed.

[0106] Next, in Step 301, the fuel injection time Ti is calculated by the following equation. $\begin{matrix} {{Ti} = {K \cdot \left( {{Qa}/{Ne}} \right) \cdot {TGFBA} \cdot {ALPHA} \cdot {Kr}}} \\ {= {K \cdot \left( {{Qa}/{Ne}} \right) \cdot {Kr}}} \end{matrix}$

[0107] In next Step 302, if output Vo of the second air/fuel ratio sensor 25 exceeds VS2 is examined. If not exceeding, if Vo exceeds VS1 is examined in next Step 302. If not exceeding VS1, release or reduction of NOx is not started (stored oxygen is released or reduced), and therefore, this control flow is terminated. In case of exceeding VS1, NOx is being released or reduced, and therefore, in next Step 304, T2 is added ΔT (control start period) (may be added 1 by 1). Next, in Step 305, an accumulated value SQa of an air flow rate Qa and an accumulated times counter CQa are respectively updated.

[0108] Where in Step 302, Vo exceeds VS2, the release or reduction of NOx is terminated, and therefore, the procedure proceeds to Step 306 for termination process. At that time, T2 is the value obtained by measuring the time from VS1 to VS2. In Step 306, the NOx purge request flag is cleared (=0), and in next Step 307, an average air flow rate Qave during the release of NOx is calculated by the following equation.

Qave=SQa/CQa

[0109] In next Step 308, the NOx trapping quantity detected value TNOD is calculated by the following equation.

TNOD=k′·Qave·Kr·T2

[0110] In next Step 309, the NOx saturated trapping quantity TNOAMX is updated according to TNOD, and the threshold TNOAP for starting the normal NOx purge control is also updated. Specifically, the following equation is used.

TNOAMX=TNOD

TNOAP=kp·TNOD

[0111] where Kp is a constant, which is a value of 0.6 to 0.8.

[0112] In next Step 310, TNOD, TNOA, T2, SQa, and CQa are initialized.

[0113] In next Step 311, if TNOAMX is smaller than the deterioration judgment threshold KNOASL is examined. If being small, in Step 312, a deterioration judgment flag is set (=1), and if being not small, in Step 312, the deterioration judgment flag is cleared (=0), after which this control flow is terminated. Where the operating mode before starting of the NOx purge control is a stratified operating mode, the control for switching the operating mode from the homogenous operating mode to the stratified operation is executed, after which this control flow is terminated.

[0114] Where the deterioration judgment flag is set, a code representative of a deterioration of the NOx trap 15 is stored by control not shown, and the warning to an operator such as turning on of an alarm lamp is executed.

[0115] While the foregoing comprises one embodiment in connection with means for diagnosing the NOx trap for trapping NOx in the exhaust components, it is noted that the similar application can be made for the HC trap for trapping HC. Further, the diagnosis of trap materials such as the NOx trap and MC trap are carried out under the conditions suitable for diagnosis to thereby enhance the accuracy thereof. In other words, where the conditions suitable for diagnosis are not provided, not-execution of diagnosis can be said the procedure for preventing erroneous diagnosis. In the following, the procedure for preventing erroneous diagnosis in diagnosing the performance of the trap material such as the NOx trap will be described.

[0116]FIG. 15 shows the conditions for executing diagnosis of the trap material. In Step 1501, the judgment of the engine system is carried out to judge if the state is a state of exhaust gas input in the trap material to be diagnosed, for example, a state in which the air/fuel ratio, the exhaust gas quantity, the exhaust gas component or the like are suited for diagnosis. That is, where the state of exhaust input in the trap material is changed, there is the possibility of exerting bad influence on the diagnostic result to result in erroneous diagnosis. The contents of Step 1501 contemplated include abnormalities of the air flow sensor 3, the throttle valve opening-degree sensor 25, the cylinder pressure sensor 12, the first air/fuel ratio sensor 14, the crank angle sensor 17, the accelerator sensor 19, etc., and in addition, abnormalities of the fuel pressure, the ignition timing, the supplied fuel quantity (for example, mistake in the air/fuel ratio every cylinder), the combustion pressure, the exhaust temperature, the EGR system, and the performance of a catalyst installed at the upstream of the trap material. Further, if the number of engine speed, the engine load and so on are states suited for diagnosis are also judged. This is because of the fact that when the exhaust gas quantity exceeds a fixed range, too large or too small, there is the high possibility of exerting influence on the result of diagnosis of the trap material also. FIG. 16 shows an example of a flow chart for judging if the state of the engine system is a state suited for the diagnosis of the NOx trap.

[0117] Where in Step 1502 in FIG. 15, judgment is made that the engine system is abnormal as described above, the evaluation of performance of the trap material in Step 1507 is bypassed. Further, in Step 1503, judgment is made if the trap material itself is a state suited for the evaluation of performance. As the element thereof, there is, for example, the temperature of the trap material, but as the actual applied example, the temperature of exhaust gas at the upstream or downstream of the trap material is measured indirectly, or the temperature of the inside of the trap material is measured directly to judge the state of the trap material. Specifically, where the temperature of the tap material measured directly or indirectly is not within the fixed range (300° C. to 400° C. in the present embodiment, though being different depending on the quality of the trap material), Step 1507 is bypassed. As another element, the another performance of the trap material (for example, such as the conversion efficiency of exhaust component different from that to be trapped) is taken into consideration.

[0118] Finally, in Step 1505, where detection is made that the diagnostic means for the trap material is not suited for diagnosis, Step 1507 is bypassed.

[0119] While a description has been made of the form of the present invention taking an example of the cylinder injection type gasoline engine, the form is not limited thereto. Even the port injection type gasoline engine, or even the Diesel engine, the judgment method for the NOx trap by means of an air/fuel ratio sensor at the downstream of the NOx trap which comprises the essential pat of the present invention can be applied. Further, while in the present invention, the performance evaluation of the trap material under the circumstances not suited for the performance evaluation of the trap material is inhibited or stopped, the result of the performance evaluation can be corrected depending on the circumstances. Further, while in the present invention, the NOx trap is mainly referred to, it is to be noted that the procedure of the present application can be also applied, for the purpose of enhancing the diagnostic accuracy, to the HC trap for adsorbing or absorbing HC of the exhaust gas component.

[0120] According to the present invention, since the release of oxygen stored and the timing of the release of NOx trapped are detected separately by the output of the air/fuel ratio sensor at the downstream of the NO trap for trapping NOx, it is possible to provide an engine exhaust purifying apparatus capable of detecting the NOx trap quantity and the oxygen storage capacity separately with good accuracy. Under the circumstances that the aforesaid detection accuracy cannot be secured, it is possible to prevent an erroneous detection by inhibiting or stopping the detecting operation. 

What is claimed is:
 1. An engine exhaust purifying apparatus, comprising: an exhaust component trap which is provided in an exhaust passage of an engine and having a trapping function to adsorb or absorb an exhaust component; and means for evaluating the performance of said exhaust component trap; wherein when at least one of an operating state of an engine system, the operating state of said exhaust component trap, and said means for evaluating the performance of said exhaust component trap is judged to be beyond a predetermined operating range, the performance evaluation of said exhaust component trap is inhibited or stopped.
 2. An engine exhaust purifying apparatus, comprising: a NOx trap which is provided in an exhaust passage of an engine to trap NOx in exhaust gases by adsorption or absorption when the air/fuel ratio of a mixture gas is lean, and to release or reduce NOx when the air/fuel ratio is rich; and NOx trapping quantity judging means for evaluating the exhaust purifying performance including the NOx trapping quantity of said NOx trap; wherein the operating state of said NOx trap is measured directly or indirectly, and when said measured operating state is judged to be beyond a predetermined range, the purifying performance evaluation of said NOx trap is inhibited or stopped.
 3. The engine exhaust purifying apparatus according to claim 2, wherein as the operating state of said NOx trap, the internal temperature of said NOx trap is measured directly or indirectly, and when said measured temperature is judged to be beyond a predetermined range, the purifying performance evaluation of said NOx trap is inhibited or stopped.
 4. The engine exhaust purifying apparatus according to claim 2, wherein the internal air/fuel ratio of said NOx trap is measured directly or indirectly, and when said measured air/fuel ratio is judged to be beyond a predetermined range, the purifying performance evaluation of said NOx trap is inhibited or stopped.
 5. The engine exhaust purifying apparatus according to claim 2, wherein the component ratio of the internal exhaust gases of said NOx trap is measured directly or indirectly, and when said measured component ratio is judged to be beyond a predetermined range, the purifying performance evaluation of said NOx trap is inhibited or stopped.
 6. An engine exhaust purifying apparatus, comprising: a NOx trap which is provided in an exhaust passage of an engine to trap NOx in exhaust gases by adsorption or absorption when the air/fuel ratio of a mixture is lean, and to release or reduce NOx when the air/fuel ratio is rich; and NOx trapping quantity judging means for evaluating the exhaust purifying performance including the NOx trapping quantity of said NOx trap; wherein an operating state of an engine system is measured directly or indirectly, and when said measured operating state is judged to be beyond a predetermined range, the purifying performance evaluation of said NOx trap is inhibited or stopped.
 7. The engine exhaust purifying apparatus according to claim 6, wherein as the operating system of said engine system, when an engine parameter influencing on at least one of the exhaust temperature of the engine, the exhaust air/fuel ratio, the exhaust component, and the exhaust gas quantity is judged to be beyond a predetermined range, the purifying performance evaluation of said NOx trap is inhibited or stopped.
 8. An engine exhaust purifying apparatus, comprising: a NOx trap which is provided in an exhaust passage of an engine to trap NOx in exhaust gases by adsorption or absorption when the air/fuel ratio of a mixture is lean, and to release or reduce NOx when the air/fuel ratio is rich; and NOx trapping quantity judging means for evaluating the exhaust purifying performance including the NOx trapping quantity of said NOx trap; wherein the operating state of said NOx trap performance evaluation means is judged directly or indirectly, and the purifying performance evaluation of said NOx trap is inhibited or stopped on the basis of said judged operating state. 